Image processing apparatus

Abstract

The image processing apparatus of the present invention determines the number of surrounding pixels to be averaged and their locations, determines a weighting coefficient based on the density and contrast levels of an input image data derived from a continuous tone image, and performs an averaging operation on the input image signal by performing a weighting operation using the averaged result of the surrounding pixels, the input image data, and the weighting coefficient. Therefore, the image processing apparatus of the present invention is capable of reducing the noise level over a wide range of image types, from shadow regions with high density. Further to highlight regions with low density, and images reproduced by this apparatus do not exhibit a deterioration of the sharpness of image boundaries in the high density regions, images characterized by a gradual change in the density level do not turn out having an unnatural quality.

What is claimed is: 
     
       1. An image processing apparatus that performs an averaging operation of input image data of a current pixel and a plurality of input image data of pixels surrounding the current pixel which are derived from a continuous tone image, and outputs corrected image data that are reduced in noise level, said image processing apparatus comprising: an averaging means for applying an averaging operation on said input image data of the current pixel and said input image data of the surrounding pixels to take an average data;   an averaging-type determining means for determining the surrounding pixels to be included in the averaging operation, based on density and contrast levels of the input image data of the current pixel;   a weighting coefficient determining means for determining at least one weighting coefficient based on the density and contrast levels of the input image data of the current pixel; and   a weighting means for performing a weighting operation on said average data and the input image data of the current pixel with the weighting coefficient to take the corrected image data.   
     
     
       2. An image processing apparatus according to claim 1 wherein said weighting means is a weight-and-add means. 
     
     
       3. An image processing apparatus according to claim 1 wherein said averaging-type determining means determines the number of pixels and the corresponding pixels to be averaged, based on the density and contrast levels. 
     
     
       4. An image processing apparatus according to claim 3 wherein said averaging-type determining means includes an averaging-type selection table which has previously stored the number of pixels and the corresponding pixels based on the density and contrast levels. 
     
     
       5. An image processing apparatus according to claim 1 wherein said weighting coefficient determining means includes a weighting coefficient selection table which has previously stored the weighting coefficient based on the density and contrast levels. 
     
     
       6. An image processing apparatus according to claim 1 wherein said averaging-type determining means is one that selects previously determined average data obtained through an averaging process that includes a predetermined set of surrounding pixels. 
     
     
       7. A method for producing corrected image data that are reduced in noise level, comprising the steps of: (a) receiving input image data of a scanning line containing a current pixel and a previous pixel and input image data of at least one scanning line other than the scanning line containing the current pixel;   (b) producing a density signal from the data of the current pixel;   (c) producing a contrast signal based on the data of the current pixel and the previous pixel;   (d) selecting at least one weighting coefficient based on the density signal and the contrast signal;   (e) determining a number of surrounding pixels to be averaged based on the density signal and the contrast signal; and   (f) averaging the data of the current pixel with the data of the surrounding pixels determined in step (e) in accordance with the weighting coefficient.   
     
     
       8. A method according to claim 7 wherein the other scanning lines correspond to a previously scanned line. 
     
     
       9. A method according to claim 7 wherein step (e) comprises selecting from a number of predetermined sets of surrounding pixels. 
     
     
       10. A method according to claim 7 wherein said determining step (e) operates such that when the level of the contrast signal decreases and the level of the density signal increases, the number of surrounding pixels averaged increases. 
     
     
       11. A method according to claim 10 wherein said selecting step (d) selects a weighting coefficient for the current pixel, and operates such that when the level of the contrast signal decreases and the level of the density signal increases, the weighting coefficient for the current pixel decreases. 
     
     
       12. A method according to claim 7 wherein said selecting step (d) selects a weighting coefficient for the current pixel, and operates such that when the level of the contrast signal decreases and the level of the density signal increases, the weighting coefficient for the current pixel decreases. 
     
     
       13. An apparatus for producing corrected image data that are reduced in noise level, comprising: an input unit for receiving input image data of a scanning line containing a current pixel and a previous pixel, and input image data of at least one scanning lines other than the scanning line containing the current pixel;   a density signal producing circuit, operatively connected to said input unit, for producing a density signal from the data of the current pixel;   a contrast signal producing circuit, operatively connected to said density signal producing circuit, for producing a contrast signal based on the data of the current pixel and the previous pixel;   a look-up table unit, operatively connected to said density signal producing circuit and said contrast signal producing circuit, for producing a weighting coefficient in accordance with the density signal and the contrast signal, and for producing a value representing a number of surrounding pixels to be averaged; and   an averaging unit, operatively connected to said input unit and said look-up table unit, for averaging the current pixel with the number of surrounding pixels to be averaged in accordance with the weighting coefficient.   
     
     
       14. An apparatus according to claim 13 wherein said look-up table unit comprises: a first look-up table containing a plurality of weighting coefficients arranged in accordance with the density signal and the contrast signal; and   a second look-up table containing values representing a number of surrounding pixels to be averaged, the values being arranged in accordance with the density signal and the contrast signal.   
     
     
       15. An apparatus according to claim 13 wherein said averaging unit comprises: an averaging circuit for averaging the number of surrounding pixels to be averaged to produce an average quantity; and   a synthesis circuit for performing a weighted averaging operation based on the current pixel, the average quantity and the weighting coefficient to produce a weighted average quantity as a corrected pixel of the corrected image data.   
     
     
       16. A method according to claim 13 wherein said look-up table unit operates such that when the level of the contrast signal decreases and the level of the density signal increases, the number of surrounding pixels averaged increases. 
     
     
       17. A method according to claim 16 wherein said look-up table unit selects the weighting coefficient for the current pixel, and operates such that when the level of the contrast signal decreases and the level of the density signal increases, the weighting coefficient for the current pixel decreases. 
     
     
       18. A method according to claim 13 wherein said look-up table unit selects the weighting coefficient for the current pixel, and operates such that when the level of the contrast signal decreases and the level of the density signal increases, the weighting coefficient for the current pixel decreases.